The present invention relates to communications systems in general and, more specifically, to methods and apparatus for reducing data loss on a network with an unreliable physical layer.
Many frame-handling schemes and protocols have been used for data transport. In systems implementing those schemes and protocols, data is “packaged” into frames and sent from a sender to a receiver (or to a plurality of receivers in a broadcast mode). When the receiver receives a frame, it unpackages the data and uses it. In the standardized ISO seven-layer network, a transmitting application moves data to a receiving application by providing the data to a layer below an application layer, which in turn provides it to the next lower layer, until the data reaches the physical layer and is sent to the receiver. At the receiver, the process is reversed, with the data flowing up from the receiver's physical layer to its application layer. The seven layers, from the bottom to top are: 1) the physical layer, 2) the link layer, 3) the network layer, 4) the transport layer, 5) the session layer, 6) the presentation layer, and 7) the application layer. FIG. 1 illustrates this concept.
Much has been written on the benefits of such an arrangement of layers and need not be recounted here. Many standards for data communications between peer layers have also been proposed and are in common use. The most common form of networked communication in a local area network in use today is Ethernet/IEEE-802.3 network protocol communication (“Ethernet” for short). Because of its ubiquity, many Ethernet components are readily available at low cost. For example, single chips are readily available that perform much of the logic needed to implement Ethernet communications. Furthermore, because of their ubiquity, those chips have been widely tested and the technology is mature to the point where its usefulness and limitations are well known, in addition to being readily available.
The Ethernet link layer was designed to be supported by a relatively reliable physical layer below the link layer. Because of this, the Ethernet link layer, in its standard operating mode, does not correct errors in frames nor does it retransmit lost frames. A number of reliable physical layers have been standardized for Ethernet, including ones based on coaxial cables, high quality twisted pair, and optical fibers. These generally deliver very low error rates (e.g. less than 1 errored frame per million frames sent). Consequently, many higher layer protocols have been designed with low error rates in mind and perform poorly at higher rates. For example, streaming video applications require low error rates, as the loss of 1 frame or more in 1,000 frames can cause visible errors. As another example, the Transport Control Protocol (TCP), which is used for virtually all wide area (Internet) reliable transport, performs poorly when more than 1 frame in 20 is lost. If an unreliable physical layer were used with standard Ethernet link layer, higher layer protocols would have more lost frames.
With the growth of home networking, more and more computer users are expecting to be able to connect their home computers and other home appliances together. With the ready availability of inexpensive Ethernet components, the computer users might like to set up an Ethernet network for the interconnected home devices. However, unless the house is wired with standardized media such as CAT 5 twisted pair cables or the like, the network must be set up over existing wiring, which usually means either phone lines or power lines (unless the network is wireless). While phone lines and power lines can easily carry lower bit rate data streams, transmitting data at Ethernet rates is challenging, since neither phone lines nor power lines were designed to carry high bandwidth communications. The typical phone line is a set of twisted pairs terminated in RJ-11 sockets with no matching termination resistors and could be daisy-chained from phone jack to phone jack with many branches further degrading the lines and adding noise onto the lines. Power lines have their own set of problems with the power grid and appliances injecting noise onto the wires.
The unreliability of these lines at high bandwidths results in frame loss or delay. Where the noise or other impairments on a line cause the receiver to make even a single error in decoding a frame's data, that frame is lost. Loss of frames results in loss of data or delay due to retransmission of the lost data. For time sensitive applications, delay may pose a significant problem. For other applications, such as UDP-based multicast of audio/video streams, frequent lost frames are a problem.
Attempts to increase the reliability of an unreliable channel generally fall into two categories: using error-correcting codes (forward error correction) or using a reliable data stream protocol. Error correcting codes require that extra data be sent, using up extra channel capacity for all traffic and extra computational resources in the sender even when there are no errors. Even with strong error correcting codes, frames may still be lost because the frame is too badly damaged or is simply lost because of errors at the beginning or end of the frame. Therefore, extra effort will be expended computing and transporting extra bits that will either not be used, in the case of undamaged frames, or cannot be used, in the case of badly damaged frames.
Reliable data stream protocols, such as HDLC LAPB, provide a reliable data channel over an unreliable medium by having the receiver acknowledge receipt of frames and having the sender retransmit frames that are not acknowledged. A reliable data stream protocol is used to ensure that all frames are successfully received without regard to timeliness, and thus the protocol requires relatively frequent acknowledgments to be sent from the receiver to the sender, as well as flow control to halt the flow of data. These protocols also require explicit connection establishment between pairs of stations (a sender and a receiver) and do not support multicast applications. The above factors require extra bandwidth, may introduce delay and make the network less robust. For these reasons, the use of reliable data stream protocols has limitations as a method to increase reliability.
Therefore, a protocol and supporting methods and apparatus are needed that can reduce effective frame loss rates and delays to the level of standard Ethernet frame loss rates and delays, over a physical layer that is expected to be unreliable, and while doing so at minimal cost in terms of network bandwidth and host resources, and while preserving the connectionless, low-latency, and multicast service normally expected of Ethernet.